High pressure discharge lamp with electrolysis preventing means



Sept. 27, 1966 J. M. POMFRETT HIGH PRESSURE DISCHARGE LAMP WITH ELEGTROLYSIS PREVENTING MEANS Filed April 7. 1965 \m m m lnven cov'. John M. Pom+vett by United States Patent 3,275,885 HIGH DISCHARGE LAMP WITH ELECTRQLYSIS PREVENTING John M. Pomfrett, Kirfland, Ohio, assignor, to General Electric Company, a'corporation ofNew'York I 'FiledApr. 7, 1965,Ser.No. 446,333

1. Claim. '(Cl. 3159-330) This invention te ohis ntensit l t o dis h e lamps comprising a quartz or hard glass arc tube in which an .arc discharge takes placein gas or metalvapor at high pressure to produce visible light. The invention is more particularly concernedwith preventing electrolysis of the quartz or glass in the vicinity of the, electrode seals and especially about the. auxiliary starting electrode.

he most mmon, o m of high ntens ty amp t ho present time is the high PIq SDre mercury vapor lamp which generally comprises aquar tzarc tube enclosedwithin a glass outer jacket fitted with a screw base at oneend. he are h s Pr vide with h m onic, elfi oa electrodes at opposite ends and contains a quantity of or o e w a ar n osthor sis o i intensity lamps may use differentv gas or vapor fillings withinthe arc tube, for instance vaporizable metal halides such as sodium iodide and mercury. In some larnpsthe outer jacket is coated: internally with a phosphor in order to improve the quality or the total quantity of light emitted.

A relatively high voltage is required across the main electrodes in order to initiate the arc in high pressure lamps. In order to avoid 'thenecessity for such a high voltage, it has been the practice to provide an auxiliary starting electrode which is located, adjacent the main electrode at one endjof the lamp andconnected through a current limiting resistor to the main electrode at the other end. The possibility of deyitrification of the quart; or glass seal between starting and adjacent main electrode in high pressure metalvapor lamps is well known. It occurs when a unidirectional potential arisesbetween the starting and the adjacent main electrode byjreason ofcurrent rec tification by the starting electrode. The general solution to this pr b em s u h n Pate t. 60.69 St- L ui et al. and consists in increasing the electron, emission of the starting electrode to the point where its conductivity is substantially symmetrical on both half-cycles of the applied alternating current. Practicalconstructions proposed by the StlLouis patent for achieving this result include a long starting electrode extending substantially to the arc stream in front of the adjacent main electrode, An alternative construction provides a ring on the end of the auxiliary electrode which is located slightly forward of and surrounding the tip of the adjacent main electrode, and this was preferred for. lamps intended for"operation in any position including the horizontal, and not restricted to vertical operation. i

In current manufacture of high pressure mercury vapor lamps, foil inleads are used and they are pinch sealed into the ends of the quartz arc tube. The pinch-sealing process is fast and amenable to complete mechanization and has almost completely replaced the older vacuum sealing process which required much skilled hand la-bor. Unfortunately with pinch sealing it is very difficult to position the electrodes accurately. At the instant just prior to sealing, the electrode is supported entirely by the very thin and flexible molybdenum'foil. The foils at sealing temperature do not have sufiicient rigidity even to support the electrodes upright and they are straightened out only when the plastic quartz is pressed into engagement: thus positioning cannot be accurate. This has made the ringtype starting electrode preferred by the St. Louis patent for lamps intended for horizontal operation, impractical.

The object of the invention is to provide a new and a part of the luminaire.

Pa en d sop .9 6.

i llx o t o loomo t s o t odosor n e ds The invention is based upon the obseryation that elec; ro t o oo ai oro oho iz o ly pototool mps t on e t na aux ia yv le trodes e er l y. o urs h e amo sso ot od o s a s ot ho ili lo iodo is ower os he x ia e ec ode h mos o old o. -ovo o. olo' trooom s ioo. nec y o vo o' ot oot fi o ion. nd. ea e ro is. It s n t P io l t k he cke o o fi toroor ho so of e am d st b e M nv nt on fi ds. so oo o iool an co v nien o t on t o l m w i h. onsis s in P vi in wio o i xo o ss. t. n nd o h arc tube and located on opposite sides of the elecrode, The twoaosil l c odes ar i k d o e he and a o n e to h mo o stro o at h p o it end, of, the lamp through the starting resistorin the usual way. With this arrangement, there is always one of the aux lia y l d s o s oni n a v he x o h arc tube so that it is sufliciently heated by the discharge to achieve the electron emissivity necessary for avoiding e lectrolysis of the seals.

or. o h io ts nd, d anta s ds o e r: an h e tio t en i n is h w sotoo to h following c i t o v n oo m 'ony h r w n us ating a prefered embodiment. The features believed to be uoy l ill o more P c lar y Po o o t n t oo n d =o1 im.-'

In the: drawing:

E16. 1 s s i e el va io Pa e oned of a mos: cury vapor lamp luminaire in which the lamp is seen h izontally moun d.-

FIG. 2 is a side eleyation of. the arc tube of, the lamp provided with twin auxiliary starting electrodes at one endin accordance with the invention.

Re errin t G- o raw n r s shown. a high pressure, mercury vapor lamp 1 horizontally mounted within .a luminaire 2 comprising aninne rellector 3 with:- in an out r c i 4 ppo t iby a rack t 5.- The amp comprises an outer vitreous envelope or jacket 6 of generally ovoid or ellipsoidal shape and made of soft glass, or if preferred of a hard borosilicate glass. The neck 7 of the outer. enve p is losed b a oroht h s em *5 having a press 9., through which extend stifi inlead wires 11', 12 connected into the-usual screw type base 13, one to the threaded shell and the other to the insulated center contact 14. Base 13 of the lamp is screwed into a conventional screw type socket 15 which is a permanent In the manufacture or in the assembly of the luminaires and sockets or inthe selection of lampsfor the socket, no attempt is made to assure a particular orientation of the lamp when it is screwed'into the socket and it would not be practical to do so.

The inner arc tube 17 is made of quartz' or fused silica and is provided at opposite ends with a pair of main discharge supporting electrodes, 18 at the base end and 19 at the outer end. The electrodes are mounted on inleads 20, 21 having intermediate thin foil portions which are hermetically sealed through full diameter pinch seals 22, 23 at the ends of the arc tube. Each main electrode comprises a core portion which is a prolongation of the inner end of the inlead made of tungsten and surrounded by a double tungsten wire helix. The turns of the helix and the interstices between turns are coated or filled with an electron-emitting mixture of alkaline earth oxides. The are tube is supported the outer jacket by means of a harness comprising a side rod 24 which is anchored to one of the inleads through the stem at one end and engages a re-entrant dimple 25 at the domed end of the bulb. The arc tube is supported in the harness by clamping the pinched ends between metal bands 26,27 which extend between side rod' 24' and complementary end portions 28,29. Electrode 18 is connected directly to one of: the inleads 11, 12 while electrode 19 is con: nected to the other inlead through side ro'd 24.

Inaccordance with the invention, two auxiliary elec-.

trodes 31, 32 of refractory wire are'provided at one'end of the arc tube and located on opposite sides of the main electrode at that end. As shown in FIG. 2, the auxiliary electrodes are located at the base end of the arc'tube on either side of'main electrode 18. Suitably thevauxiliary electrodes mayconsist of tungsten wires forming the electrodes proper and projecting into the arc chamber.

These wires are welded to the foliated ends of molybdenum inlead wires 31, 32' which are embedded in the quartz pinch and make the hermetic seal. Desirably the auxiliary. starter electrodes should project into the arc tube approximately coextensively with the main electrode; in the illustrated embodiment they are about even with the tip of the main electrode 18; The lateral. spacing of the auxiliary electrodes from the main electrode should be-the.

least possible consistent with the requirement that sparking should be avoided and actual contact must never take place. Since in pinch sealing the accuracy of location Y and-orientation of the foilsj within the pinchand the electrodes which are attached thereto is never too great, this imposes practical. limits on the proximity of auxiliary electrodes to main electrode to which'the arc-tube must s 4; I the starting electrode and the adjacent main electrode, which causes electrolysis to occur in the quartz between their respective inleads at the high temperature of operation. The provision of two starting;clectrodes in. accordance with the inventionavoids the foregoingpossibility by insuring that at least'oue of the auxiliary electrodesis always hotenough to provide the requisite electron emission. Actual .lifel tests conducted on mer' cury vapor lamps provided with twin auxiliary starting electrodes at one end: as described, and operatedhon'e zontally with random orientation," have. shown that the invention eliminates. electrolysis zand auxiliary electrode seal failure as a cause. of premature lamp failure.

'While the invention has been most useful to date :in conjunction with high pressuremercury vapor lamps: and has been'so illustrated and described,'it may be used with other types of electric discharge lamps where a :similar problempf excessive cooling of the auxiliary electrode mayoccur depending upon the orientation of the lamp.

The specilic embodiment which has-been illustrated and 1 described in detail is intended as exemplary and the scope of the invention-isrto be determined by theappended I claim:

be designed. In practice for. the usual sizes of mercury vapor lamps, a design spacing of 2.5 millimeters between the edge of the helix on main electrode 18 and the projecting portions of auxiliary electrodes 31, 32 has been found to result in a range ,of spacingfrom 1.5 to 3.5 millimeters in actual manufacture. This spacing is satisfactory. for the purposes of the invention and avoidsthe frequent'rejectsproduced when too stringent tolerance is attempted.

The two auxiliary electrodes 31, 32 are electricallyconnected together through a tie-wire 33, and thence connected to the opposite main electrode .19 through the usual current limiting resistor 34, for instance a 40,000 ohm resistor in the. case of 700and 1000 watt lamps. By

providing .two starting electrodesat the same end on op posite sides of the main electrode, there is always one 1 As a result, the electron emission of the starting elec-v trode falls 01f rapidly and its effectiveness as -a cathode is greatly reduced while its effectiveness as an aHOdCIS not appreciably altered. The result is a tendency for a .direct current component of voltage to develop between "What I claim as new and desire to secure by Letters Patent otthe'United States is:

-A discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube, said are tube having a pair of main thermionic activated electrodes sealed therein at opposite ends, an ionizable filling within said arc, tube including mercury and .subject to convectionefiects during horizontal operation-cf said lamp, said main electrodes being provided with external connections through seals at opposite ends of said are tube, and a pair ,of auxiliary tungsten wirestarting electrodes located adjacent one of said main electrodes and extending approximately coextensive therewith on either side vand provided with externalconnections t-hrough seals generally side-by-side with that ofsaid adjacent main electrode, saidauxiliary. 1 electrodes being connected together and joined to the main 1 electrode at the opposite end of the arc tube through a v current limiting resistor, whereby one of said electrodes is i always located on or above the level-of theyhorizo'ntal plane through the adjacent main electrode irrespectively of the orientation of said lamp, saidone electrode being sufliciently heated by the discharge to achieve atall times.

electron emission adequate to prevent electrolysis in the quartz in the vicinity of the seals.

References Cited by the. Examiner UNITED STATES PATENTS 1,930,087 10/1933. Forney 313-198 X 2,671,183 3/1954 I St. Louis. 2,845,557 7/1958 Gottschalk .313'198 X JAMES w. LAWRENCE, Primary Examiner.

GEORGEN. WESTBY, DAVIDJ.*GALV1N,-

7 1 Examiners. S. SCHLOSSER,'Assistant Examinen: 

